Additive method enabling air plasma spray (aps) over a smooth surface

ABSTRACT

A stacked up structure can include a first environmental barrier coating (EBC) layer and a second EBC layer. A first process can be used to form the first layer and a second process can be used to form the second layer. In one embodiment interfacial material can be formed for improved bonding of the second layer to the first layer. The interfacial material can define a continuous or discontinuous layer of nonuniform thickness.

FIELD

The disclosure relates to structures in general and in particular to structures of stacked up construction.

BACKGROUND

Stacked up construction methods are used in a variety of applications for manufacture of equipment parts. Stacked up structures can include a variety of materials. In one example a stacked up structure can include ceramic matrix composites (CMCs). In one example a stacked up structure can include, e.g., rare earth material. Rare earth material can include, e.g., rare earth disilicates and rare earth monosilicates. Rare earth disilicates can be particularly useful for providing hermeticity. Rare earth monosilicates can be particularly useful for providing recession resistance. Stacked up structures can include environmental barrier coatings (EBCs) for CMCs. Environmental barrier coatings can include one or more rare earth material layer.

BRIEF DESCRIPTION

A stacked up structure can include a first environmental barrier coating (EBC) layer and a second EBC layer. A first process can be used to form the first layer and a second process can be used to form the second layer. In one embodiment interfacial material can be formed for improved bonding of the second layer to the first layer. The interfacial material can define a continuous or discontinuous layer of nonuniform thickness.

DRAWINGS

FIG. 1 is a flow diagram illustrating a method for fabrication of a stacked up structure;

FIG. 2 is a schematic diagram of a stacked up structure;

FIG. 3 is a schematic diagram of a stacked up structure;

FIG. 4 is a schematic diagram of a stacked up structure;

FIG. 5 is a schematic diagram of a stacked up structure.

FIG. 6 is a schematic diagram illustrating interfacial material defining a continuous layer;

FIG. 7 is a schematic diagram illustrating interfacial material defining a discontinuous layer;

FIG. 8 is a schematic diagram illustrating interfacial material in the form of particles embedded in a first layer and defining a defining a discontinuous layer; and

DETAILED DESCRIPTION

As set forth in FIG. 1, a method can include at block 110 forming a first layer and at block 120 forming a second layer. In one embodiment, the first layer and the second layer can be environmental barrier coating (EBC) layers. In one embodiment, the first layer and the second layer can include rare earth disilicate (REDS) material. In one embodiment, the first layer can be formed using a first process and the second layer can be formed using a second process.

A stacked up structure 10 formed using the method of FIG. 1 can include a first layer 210 and a second layer 220 as shown in FIG. 2. In one embodiment, the first layer 210 can be formed using a first process and the second layer 220 can be formed using a second process. In one embodiment, first layer 210 can include first properties and the second layer 220 can include second properties. In one embodiment, each of a first layer 210 and second layer 220 can be an EBC layer.

Embodiments herein recognize that there may be challenges to bonding first and second EBC layers. Embodiments herein recognize that bonding between EBC layers can be improved by providing interfacial material. Particular embodiments herein further recognize that it can be particularly challenging to bond a second rare earth disilicate layer to a first rare earth disilicate layer where the second rare earth discilate layer is formed using APS process, and where the first rare earth discilate layer is formed using a slurry-based process.

Another embodiment of a stacked up structure 10 is shown in FIG. 3. In the embodiment of FIG. 3, stacked up structure 10 can include interfacial material 215 between the first layer 210 and the second layer 220. In one embodiment of stacked up structure 10, first layer 210 can include a first rare earth disilicate material and second layer 220 can include of a second rare earth disilicate material. Between the first layer 210 which can be a first disilicate layer and a second layer 220 which can be a second disilicate layer there can be provided interfacial material 215. The interfacial material 215 can be of nonuniform thickness. In one embodiment, interfacial material 215 can be provided by a continuous layer of material. In one embodiment, interfacial material 215 can be provided by a discontinuous layer of material that defines voids where the layer is absent of interfacial material.

Referring to the embodiments of FIGS. 4 and 5, a stacked up structure 10 can also include substrate 180, layer 190, and layer 230. Substrate 180 in one embodiment can include a ceramic matrix composite (CMC), layer 190 can be a bond coat layer including silicon, and layer 230 can be a rare earth monosilicate (REMS) layer including a rare earth monosilicate (REMS) material. In one embodiment, bond coat layer 190 can include elemental silicon, metal silicon or combinations thereof. Layer 230 can define a top layer of structure 10. Referring to layers 210, 215, 220, 230, layers 210, 215, 220, and 230 can be regarded as EBC layers, and layers 210, 215, 220 and 230 (or a subset of such layers) can be regarded collectively as an EBC coating. As indicated in the above embodiments of FIG. 4 and FIG. 5, an EBC coating can be used to protect a CMC substrate 180 defining an article.

Referring to the embodiments of FIG. 3-5, interfacial material 215 can improve bonding of first layer 210 and second layer 220, where first layer 210 can be a first rare earth disilicate layer and the second layer 220 can be a second rare earth disilicate layer. In one embodiment a rare earth disilicate can be used for hermeticity and coefficient of thermal expansion (CTE) matching. Rare earth monosilicates (e.g., of layer 230) can be used for providing recession resistance, i.e., resistance to an EBC material, e.g., silicon leaving an EBC coating. In one embodiment, first layer 210 can be formed using a first process and second layer 220 can be formed using a second process. In one embodiment, the first process can be a process that builds thickness at a relatively higher time cost and the second process can be a process that builds thickness at a relatively lower time cost.

Further regarding substrate 180 as set forth in the embodiments of FIGS. 4 and 5 substrate 180 can include a Ceramic Matrix Composite (CMC) material. Various methods can be used for fabrication of a CMC material. CMC material can be fabricated by melt infiltration (MI). Using MI, a preform can be placed in a chamber and arranged to be in contact with a source of liquid silicon. Liquid silicon can react with material of the preform. A CMC structure can also be fabricated using chemical vapor infiltration (CVI). For performance of CVI, a preform can be placed in a vapor chamber to cause a reaction between vapor of the chamber and material of the preform. A CMC structure can also be fabricated using polymer impregnation and pyrolysis (PIP). For performance of PIP, polymeric silicon carbide precursor can be used to infiltrate a fibrous preform. A CMC structure can also be fabricated using a combination of processes.

Layer 190 as set forth embodiments of FIGS. 4 and 5 can be a bond coat layer. Layer 190 can be of smooth texture (e.g., can have a relatively low roughness) and can bond to substrate 180. Layer 190 can facilitate bonding between substrate 180 and one or more rare earth material layer.

A method of fabricating structure 10 as set forth in the embodiments of FIG. 4 and FIG. 5 can include forming layer 190 on substrate 180, forming layer 210 (block 110, FIG. 1) on layer 190, forming interfacial material 215 on layer 210, forming layer 220 (block 120, FIG. 1) on interfacial material 215 or a surface defined by interfacial material 215 and layer 210 and forming layer 230 on layer 220.

Layer 210 as set forth in the embodiments of FIGS. 2-5 can be a first rare earth disilicate layer, and in one embodiment can be formed using a slurry process. In one embodiment, layer 210 can be formed using a slurry process by applying a single pass slurry to layer 190. In one embodiment, layer 210 can be formed using a slurry process by applying a multiple pass slurry to layer 190. Layer 210 can be formed, e.g., by dipping or spraying. Layer 210 in one embodiment can include rare earth disilicate material and layer 210 in one embodiment can provide hermetic sealing functionality. Layer 210 can be adapted to readily bond to layer 190 which can be a bond coat layer.

Layer 220 as set forth in the embodiments of FIGS. 2-5 can be a second rare earth disilicate layer, layer 220 in one embodiment can be formed using an air plasma spray (APS) process. Using an APS process, thickness of structure 10 can be increased at low time cost relative to a slurry process for increasing thickness using a slurry process. Layer 210 can provide a hermetic seal in one embodiment. Layer 210 can provide a thermal barrier layer in one embodiment. Layer 220 can add thickness to structure 10 at relatively low time cost.

Layer 210 as set forth in the embodiments of FIGS. 2-5 can include first properties and layer 220 as set forth in the embodiments of FIGS. 2-5 can include second properties. In one embodiment, first rare earth disilicate layer 210 can have a sintered microstructure and second rare earth disilicate layer 220 can have a splat microstructure. A sintered microstructure for layer 210 can result from use of a slurry process for formation of layer 210. A splat microstructure for layer 220 can result from use of APS process for formation of second rare earth disilicate layer 220.

Interfacial material 215 as set forth in the embodiments of FIGS. 3-5 can define a bond surface that facilitates improved bonding of layer 210, which can be a first rare earth disilicate layer and layer 220, which can be a second rare earth disilicate layer.

According to one exemplary process for forming interfacial material 215, a spray process can be used to form interfacial material 215.

A method for forming interfacial material 215 can include spraying a slurry to layer 210 which can be a first rare earth disilicate layer in a manner that the slurry partially evaporates prior to reaching a surface on which it is applied. In one embodiment, the slurry can be an aerosol slurry. When slurry material does land on a surface of layer 210 there can be sufficient amounts of slurry to bond to the surface of layer 210, but not a sufficient amount of slurry to flow and form a uniform thickness coating. Slurry can be applied to a surface of layer 210 in the form of droplets, using a partially evaporating spray process as set forth hereto. Forming slurry using an aerosol spray process can include controlling the aerosol spray process so that an applied slurry does not form uniformly on a surface of layer 210. After interfacial material 215 is formed on layer 210, the stacked up structure 10 in an intermediate stage of fabrication can be heat treated to increase bonding between interfacial material 215 and layer 210.

Interfacial material 215 formed by spraying a slurry can be provided to be continuously formed on layer 210 and in one embodiment interfacial material 215 can define a discontinuous layer that has voids.

FIG. 6 is an exploded schematic view of the Region A of any one of FIG. 3, FIG. 4 or FIG. 5 in one embodiment and illustrates an embodiment of interfacial material 215 wherein interfacial material 215 is continuously formed on layer 210. In the example shown in FIG. 6 interfacial material 215 can be formed by spraying a slurry, e.g., an aerosol slurry to layer 210 provided by a first rare earth disilicate layer including a rare earth disilicate material in a manner that the slurry partially evaporates prior to reaching a surface on layer 210. When a slurry material does land on a surface of layer 210 there can be sufficient amounts of slurry to bond to the surface of layer 210, but not a sufficient amount of slurry to flow and form a uniform coating. Nonetheless, interfacial material 215 can define a continuous layer as shown in FIG. 6. In the example described with reference to FIG. 6 slurry can be applied to a surface of layer 210 in the form of droplets, using a partially evaporating spray process as set forth hereto. In the example described with reference to FIG. 6, forming slurry using an aerosol spray process can include controlling the aerosol spray process so that an applied slurry does not form uniformly on a surface of layer 210. After interfacial material 215 is formed on first layer 210, the stacked up structure 10 in an intermediate stage of fabrication can be heat treated to increase bonding between interfacial material 215 and layer 210.

FIG. 7 is an exploded schematic view of the Region A of any one of FIG. 3, FIG. 4 or FIG. 5 in one embodiment and illustrates an embodiment of interfacial material 215 wherein interfacial material formed by spraying an aerosol slurry is discontinuously formed on layer 210 to define a discontinuous layer having voids 216. In the example shown in FIG. 7 interfacial material 215 can be formed by spraying a slurry, e.g., an aerosol slurry to layer 210 provided by a first rare earth disilicate layer including a rare earth disilicate material in a manner that the slurry partially evaporates prior to reaching a surface on layer 210. When slurry material does land on a surface of layer 210 there can be sufficient amounts of slurry to bond to the surface of layer 210, but not a sufficient amount of slurry to flow and form a continuous coating. In the example described with reference to FIG. 7 slurry can be applied to a surface of first layer 210 in the form of droplets, using a partially evaporating spray process as set forth hereto. In the example described with reference to FIG. 7, forming slurry using an aerosol spray process can include controlling the aerosol spray process so that an applied slurry does not form continuously on a surface of layer 210. Hence, voids 216 can be defined by interfacial material 215. After interfacial material 215 is formed on layer 210, the stacked up structure 10 in an intermediate stage of fabrication can be heat treated to increase bonding between interfacial material 215 and layer 210.

FIG. 8 is an exploded schematic view of the Region A of any one of FIG. 3, FIG. 4 or FIG. 5 in one embodiment and illustrates an embodiment of interfacial material 215 wherein a sprinkling (tumbling) process is used so that particles forming interfacial material 215 can be formed on layer 210. In one embodiment, illustrated in FIG. 8 particles forming interfacial layer can be formed on layer 210 by being at least partially embedded in layer 210. A sprinkling process can include sprinkling particles forming interfacial material 215 on a surface of layer 210 with layer 210 in a wet state. After layer 210 is formed, layer 210 can be in a wet state. With layer 210 being in a wet state, particles forming interfacial material 215 as shown in FIG. 8 can be sprinkled on a surface of layer 210, and later dried. In the embodiment of FIG. 8 particles forming interfacial material 215 between layer 210 and layer 215 can be partially embedded in layer 210. Drying can be performed using applied heat. On completion of drying, interfacial material 215 can have a nonuniform thickness and can define a discontinuous layer as shown in FIG. 8. In one embodiment, on completion of drying particles forming interfacial material 215 can have a nonuniform thickness and can define a discontinuous layer having voids 216. After interfacial material 215 is formed on layer 210, the stacked up structure 10 in an intermediate stage of fabrication can be heat treated to increase bonding between interfacial material 215 and layer 210.

In one embodiment, particles forming interfacial material 215 as shown in FIG. 8 can be sprinkled on a surface of layer 210 and which can be of a common material with particles of a slurry used to form layer 210. In one embodiment, particles that can be sprinkled for forming of interfacial material 215 can be of a material different than particles of slurry used to form layer 210. In one embodiment, particles used to form interfacial material 215 can be of one or more specific geometry, e.g., high aspect ratio, chopped cylinder, spherical. In one embodiment, particles forming interfacial material 215 as shown in FIG. 8 can include agglomerates of particles, e.g., particles formed by spray drying. Agglomerates of particles forming interfacial material 215 as set forth herein can include rare earth silicate particles and sintering aid particles.

The forming of interfacial material 215 in any of the embodiments of FIGS. 3 through 8 herein can provide bond surface roughening while avoiding subtractive roughening. According to an alternative method, a subtractive process, e.g., grit blasting or machining can be used to provide surface roughening between layer 210 and 220, namely by roughening of a top surface of layer 210. Embodiments herein recognize that surface roughening performed using subtractive processing can damage important functional aspects of one or more layer, e.g., layer 210, layer 190 and/or layer 180. For example, subtractive processing for surface roughening can reduce or destroy a hermeticity of layer 210 (as set forth in the embodiments of FIGS. 2-5) which can be provided as a hermetic coating. Subtractive processing can reduce mechanical capability of one or more layer, e.g., layer 210 (as set forth in the embodiments of FIGS. 3-5), layer 190 (as set forth in the embodiments of FIGS. 4-5), and/or layer 180 (as set forth in the embodiments of FIGS. 4-5). Embodiments herein recognize that forming layer 220 on a smooth surface defined by layer 210 without roughening of layer 210 and without formation of a roughened interfacial material 215 may result in a weak bond between layer 210 and layer 220.

Referring further to stacked up structure 10, stacked up structure 10 can include a layer 230 as set forth in the embodiments of FIGS. 4-5. Layer 230 can be provided by a rare earth monosilicate (REMS) layer that includes a rare earth monosilicate material. Layer 230 can provide smooth surface. Layer 230 can provide recession resistance.

Stacked up structure 10 as set forth in the embodiments of FIGS. 2-5 can include a bond surface on which layer 220 is bonded. In the case where stacked up structure 10 includes interfacial material 215, and where interfacial material 215 defines a continuous layer a bond surface on which layer 220 is bonded can be defined by interfacial material 215. In the case where stacked up structure 10 includes interfacial material 215, and where interfacial material 215 defines a discontinuous layer a bond surface on which layer 220 is bonded can be defined by interfacial material 215 and layer 210.

As used in the field of surface metrology, the term “surface roughness” (also, interchangeably herein, “roughness”) generally refers to a statistical expression of high-frequency deviations of surface height from a nominal baseline value, often a local mean surface height. Many different parameters may be used to describe the roughness of a given surface. Profile roughness parameters such as the arithmetic average of absolute values (Ra) and the root mean squared roughness (Rq) are commonly used parameters because they are readily measured using standard profilometry equipment and are easily calculated, though such measurements may not always provide the most useful description of a surface's roughness characteristics. Standard B46.1 of the American Society of Mechanical Engineers (ASME) provides procedures for measuring and calculating several different profile roughness parameters, including those noted above. Other types of roughness measures include parameters calculated over an area, as described in ISO 25178 published by International Organization for Standardization. Still other parameters are known and described in the literature.

For the purposes of the present description, “surface roughness” (and its abbreviated equivalent, “roughness”) will be understood to include any one or more of these parameters, wherein a surface of interest, namely, a bond surface of stacked up structure 10 has an “initial roughness” prior to forming of interfacial material 215 on structure 10, and a “processed roughness” after forming of interfacial material 215 on structure 10. In one embodiment, the roughness parameter is a profile roughness parameter such as Ra.

Referring to the embodiments of FIGS. 2-8 a roughness of a bond surface on which layer 220 is bonded is greater with interfacial material 215 present in stacked up structure 10 than with interfacial material 215 absent from stacked up structure 10. Interfacial material 215 increases the contact surface area (that is the area of the interface) along which second layer 220 is bonded to first layer 210, beyond what that contact area would be in the absence of interfacial material 215. In a cross-section projection, as in FIGS. 6-8 a bond surface on which layer 220 is bonded defines an interfacial bond line 500 along which second layer 220 contacts first layer 210 and/or interfacial material 215. In the cross-section projection of Region A illustrated in the embodiments of FIGS. 6-8 it is seen that in the absence of interfacial material 215, a bond surface on which layer 220 is bonded would define alternative interfacial bond line 502 shown in dashed form along which second layer 220 would contact first layer 210. In accordance with embodiments herein within a certain cross section projection, e.g., Region A in the various embodiments of FIGS. 6-8, bond line 500 is longer in the presence of interfacial material 215 than would be bond line 502 in the absence of interfacial material 215. Interfacial material 215 can increase the roughness of the interface between layer 220 and layer 210, beyond what that roughness would be in the absence of interfacial material 215. Interfacial material 215 can increase the roughness of a bond surface for bonding layer 220 to an under substructure of structure 10 beyond what that roughness would be in the absence of interfacial material 215.

For a given cross sectional projection of structure 10, e.g. as shown by Region A in FIGS. 6-8, interfacial material 215 can result in a length of bond line 500 being longer than a length of bond line 502 by more than a threshold percent. In one embodiment, the threshold percent is 10%. In one embodiment, the threshold percent is 20%. In one embodiment, the threshold percent is 30%. In one embodiment, the threshold percent is 40%. In one embodiment, the threshold percent is 50%. In one embodiment, the threshold percent is 60%. In one embodiment, the threshold percent is 70%. In one embodiment, the threshold percent is 80%. In one embodiment, the threshold percent is 90%. In one embodiment, the threshold percent is 100%. In one embodiment, the threshold percent is 120%. In one embodiment, the threshold percent is 140%. In one embodiment, the threshold percent is 160%. In one embodiment, the threshold percent is 180%. In one embodiment, the threshold percent is 200%. In one embodiment, the threshold percent is 250%. In one embodiment, the threshold percent is 300%. In one embodiment, the threshold percent is 400%. In one embodiment, the threshold percent is 200%. In one embodiment, the threshold percent is 500%.

In one embodiment, interfacial material 215 can have an average thickness less than an average thickness of layer 210 and/or layer 220. In one embodiment, the threshold percent is 50%. In one embodiment, the threshold percent is 40%. In one embodiment, the threshold percent is 30%. In one embodiment, the threshold percent is 20%. In one embodiment, the threshold percent is 10%. In one embodiment, the threshold percent is 5%. In one embodiment, the threshold percent is 4%. In one embodiment, the threshold percent is 3%. In one embodiment, the threshold percent is 1%. In one embodiment, the threshold percent is 1%.

In one embodiment, interfacial material 215 can have an average thickness that is less than a threshold percent of a thickness of layer 210. In one embodiment, the threshold percent is 50%. In one embodiment, the threshold percent is 40%. In one embodiment, the threshold percent is 30%. In one embodiment, the threshold percent is 20%. In one embodiment, the threshold percent is 10%. In one embodiment, the threshold percent is 5%. In one embodiment, the threshold percent is 4%. In one embodiment, the threshold percent is 3%. In one embodiment, the threshold percent is 2%. In one embodiment, the threshold percent is 1%.

In one embodiment as shown in FIG. 4 structure 10 can include a first vertical cross section i-i having a first layer profile and a second vertical cross section having a second layer profile. The first layer profile (cross-section i-i) can be characterized by layer 190 formed on substrate 180, layer 210 formed on layer 190, layer 215 formed on layer 210, layer 220 formed on layer 215, and layer 230 formed on layer 220. The second layer profile (cross-section ii-ii) can be characterized by layer 190 formed on substrate 180, layer 210 formed on layer 190, and layer 230 formed on layer 210. The second layer profile can be absent of layer 220 and layer 215. Layer 230 can be a common top layer for each of the first and second layer profiles. In one embodiment as shown in FIG. 5 structure 10 can include a first vertical cross section aa-aa having a first layer profile and a second vertical cross section bb-bb having a second layer profile. The first layer profile (cross-section aa-aa) can be characterized by layer 190 formed on substrate 180, layer 210 formed on layer 190, layer 215 formed on layer 210, layer 220 formed on layer 215, and layer 230 formed on layer 220. The second layer profile (cross-section bb-bb) can be characterized by layer 190 formed on substrate 180, layer 210 formed on layer 190, layer 215 formed on layer 210, and layer 230 formed on layer 215. The second layer profile can be absent of layer 220. Layer 230 can be a common top layer for each of the first and second layer profiles.

It was determined that layer 230 can readily bond to layer 220 as well as layer 210. Structure 10 as set forth in the embodiments of FIG. 4 and FIG. 5 can define an air foil having a leading edge 12 and a trailing edge 14. Leading edge 12 can have a first thickness and the first layer profile. Trailing edge 14 can have the second thickness and the second layer profile.

Stacked up structure 10 can be fabricated into one of a variety of useful forms. Stacked up structure 10 which can include CMC substrate 180 as well as layers 190, 210, 215, 220, 230 set forth in reference to the embodiments of FIGS. 2-8 can be provided in a form to define an article, e.g., of an airfoil, a shroud, a blade, a vane, a nozzle, a turbine center frame, a cowl, an exhaust mixer. Fabricating structure 10 to include an EBC coating can include performance and longevity of stacked up structure 10.

It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the various embodiments without departing from their scope. While the dimensions and types of materials described herein are intended to define the parameters of the various embodiments, they are by no means limiting and are merely exemplary. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the various embodiments should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. Forms of the term “defined in the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure. It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the disclosure may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

1. A stacked up structure comprising: a first rare earth disilicate layer; a second rare earth disilicate layer; and interfacial material defining a bond surface on which the second rare earth disilicate layer is bonded, said interfacial material formed between the first rare earth disilicate layer and the second rare earth disilicate layer, wherein a roughness of the bond surface is greater than a roughness of a surface for bonding the second rare earth disilicate layer in an absence of the interfacial material.
 2. The stacked up structure of claim 1, wherein the interfacial material is formed on the first rare earth disilicate layer.
 3. The stacked up structure of claim 1, wherein the interfacial material comprises particles at least partially embedded in the first rare earth disilicate layer.
 4. The stacked up structure of claim 1, further having a bond coat layer below the first rare earth disilicate layer.
 5. The stacked up structure of claim 1, wherein the interfacial material defines a continuous layer formed on the first rare earth disilicate layer.
 6. The stacked up structure of claim 1, wherein the interfacial material defines a discontinuous layer formed on the first rare earth disilicate layer.
 7. The stacked up structure of claim 1, wherein the first rare earth disilicate layer includes a sintered microstructure, and wherein the second rare earth disilicate layer includes a splat microstructure.
 8. The stacked up structure of claim 1, wherein a length of a bond line defined at the bond surface in a cross section projection of the stacked up structure is more than 20 percent longer than a bond line defined at the surface for bonding in the cross sectional projection of the stacked up structure.
 9. The stacked up structure of claim 1, wherein the interfacial material has an average thickness of less than 50 percent of an average thickness of the first rare earth disilicate layer.
 10. The stacked up structure of claim 1, wherein the bond surface is defined by the interfacial material.
 11. The stacked up structure of claim 1, wherein the bond surface is defined by the interfacial material and the first rare earth disilicate layer.
 12. The stacked up structure of claim 1, wherein the stacked up structure includes a CMC substrate, wherein the first rare earth disilicate layer is formed over the CMC substrate.
 13. A method comprising: forming a first rare earth disilicate layer on a surface; forming interfacial material on the first rare earth disilicate layer to define a bond surface; and forming a second rare earth disilicate layer on the bond surface, wherein the forming interfacial material results in the bond surface having a roughness greater than a roughness of a surface for bonding of the second rare earth disilicate layer in an absence of the interfacial material.
 14. The method of claim 13, wherein the forming a first rare earth disilicate layer includes using a first process and wherein the forming a second rare earth disilicate layer includes using a second process.
 15. The method of claim 13, wherein the bond surface is defined by the interfacial material.
 16. The method of claim 13, wherein the bond surface is defined by the interfacial material and the first rare earth disilicate layer.
 17. The method of claim 13, wherein the forming interfacial material includes using an additive process.
 18. The method of claim 13, wherein the forming interfacial material is performed so that the interfacial material defines a continuous layer.
 19. The method of claim 13, wherein the forming interfacial material is performed so that the interfacial material defines a discontinuous layer.
 20. The method of claim 13, wherein the forming interfacial material includes using spray evaporating.
 21. The method of claim 13, wherein the forming interfacial material includes sprinkling particles onto the first rare earth disilicate layer with the rare earth disilicate layer in a wet state.
 22. The method of claim 13, wherein the forming interfacial material includes sprinkling particles onto the first rare earth disilicate layer, wherein the particles include agglomerates of particles.
 23. The method of claim 13, wherein the forming a first rare earth disilicate layer includes forming a slurry, and wherein the forming a second rare earth disilicate layer includes using an air plasma spray (APS) process.
 24. The method of claim 13, wherein the first rare earth disilicate layer includes a sintered microstructure, and wherein the second rare earth disilicate layer includes a splat microstructure.
 25. The method of claim 13, wherein the forming a first rare earth disilicate layer on a surface includes forming the first rare earth disilicate layer over a CMC substrate.
 26. A stacked up structure comprising: a plurality of rare earth material layers defining an environmental barrier coating (EBC); a first vertical cross section extending through one or more layers of the plurality of rare earth material layers, the first vertical cross section having a first layer profile; and a second vertical cross section extending through one or more layers of the plurality of rare earth material layers, the second vertical cross section having a second layer profile.
 27. The stacked up structure of claim 26, wherein the second layer profile is absent a layer included in the first layer profile.
 28. The stacked up structure of claim 26, wherein the second layer profile is absent a plurality of layers included in the first layer profile.
 29. The stacked up structure of claim 26, wherein the second layer profile is absent a layer included in the first layer profile, and wherein the first layer profile and the second layer profile include a common top layer.
 30. The stacked up structure of claim 26, wherein the stacked up structure defines an article selected from the group consisting of an airfoil, a shroud, a blade, a vane, a nozzle, a turbine center frame, a cowl, an exhaust mixer.
 31. The method of claim 13 wherein the forming the first rare earth disilicate layer comprises depositing an aerosol slurry of the first rare earth disilicate, and wherein the forming interfacial material comprises spraying a slurry of the interfacial material onto the first rare earth disilicate or depositing particles of the interfacial material onto the first rare earth disilicate layer, and wherein the forming the second rare earth disilicate layer comprises air plasma spraying the second rare earth disilicate. 